As world oil supplies decrease, it is becoming increasingly desirable for companies to drill for oil at offshore locations. Exploration and production of oil from below the sea floor depend partly upon the stability of the offshore structure and existing designs utilize either a buoyant or bottom supported structures. Currently, one type of offshore oil platform is a Tension Leg Platform (TLP) which consists of a positively buoyant structure tied down by tendons that are under severe tension. TLP structures, like many others, are to be put into use for long periods of time, something on the order of 20 to 30 years or more. As a result, forces that act on these structures, such as wave, current and wind forces, constribute to fatigue and reduce the structure's ultimate life span. This fatigue may, of course, be acute or chronic, but here the discussion is limited to the chronic stress and strain generated over longer periods of time.
Wave action is considered the greatest source of structural fatigue because wave forces will cause the structure to resonate at its natural frequency. Once this happens, the initial wave force is amplified into a larger, more distinctive force which may subject the structure and tendons to higher loads than they could bear.
When a TLP encounters waves, its response may include heave, pitch, roll, yaw, surge, and sway. Heave refers to vertical movement of the buoyant body; pitch refers to rotational movement about the transverse axis; roll is movement about the longitudinal axis; surge and sway refer to horizontal movement; and yaw is rotation about the vertical axis. Of these, the three major responses we are concerned with are heave (see FIG. 4A), pitch (see FIG. 4B), and roll. The wave forces that contribute to the heave and pitch may also be divided up into two components; i.e., a primary and a secondary motion. Primary motion is that which causes the large or initial vertical heave due to the wave trough and crest displacement or to the large initial pitch due to the impact from the front of the wave (or even the structure riding the wave surface). However, secondary motion is caused by wave dynamics and is more subtle.
In wave dynamics, an ocean wave has many frequency components. Motion due to secondary wave dynamics can occur when two waves of different frequencies interact. For example, if one wave has a period of 5.0 seconds and a second wave has a period of 6.0 seconds they will interact at a given time and create two additional waves. An additive type, or beating, wave will be created with a 30-second period (this generally has an effect on surge motion) and a subtractive, or springing, wave will be created with a period of 2.75 seconds (this mostly has an effect on heave and pitch). Once these waves of varying period encounter the structure, they excite it at its natural frequencies. These natural frequencies are dependent on the structure's mass and stiffness, not the wave, and may be measured using an accelerometer.
As stated before, when these secondary wave forces act on a structure they cause, among other things, heave, pitch, and roll. Since heave is vertical movement when the platform's surface remains parallel to the sea floor, the tendons will be compressed or stretched equally. This means that positive effects from damping will be spread over all the TLP tendons. Pitch, however, is different (roll will not be discussed further as it is the same as pitch, but in a different plane.). When a structure rotates above its transverse axis the tendon stretch or compression is the pitch angle times the length from the transverse axis to the leg. As a result, small angles of pitch may be translated into large tendon tension variations with platforms of greater length. When both heave and pitch responses occur together (which is usually the case) the amount of displacement may be shown by FIGS. 4C and 5 and the equation .DELTA.H total=.DELTA.H.sub.H +.DELTA.H.sub.p. This is damaging to the anchoring apparatus and exacerbates any faults or inadequacies in the anchoring device employed. Consequently, if these secondary heave and pitch motions are left unchecked for a long period of time, chronic fatique on the offshore structure will occur.
There are some U.S. patents that deal with absorbing systems for offshore structures; however, they are either connecting devices that try to reduce a load transfer or are directed towards the primary response only and do not reduce high frequency resonant motion. They try to compensate for the larger, first order wave forces in heave, pitch, roll, or surge. Many of these designs are unnecessarily complex and expensive and do not address themselves to the resonant motion at high frequencies. As a result, the focus of the present invention is on absorbing the resonant heave and pitch responses due to secondary wave excitation and the elimination of amplification by the total acceptance of the force load by transferring it to a secondary device. It is the ever present random seastate that is important because pairs or waves are continuously combining to produce secondary forces at the natural frequencies of heave, pitch, and roll.
Consequently, it is the principal object of this invention to provide an economical and simple damping system that will absorb the secondary heave and pitch motion in an offshore structure due to wave exitation (the term "damping" as applied to Applicant's invention means a device that may accept a load from an excited body). It is another object of this invention to reduce chronic fatique on an offshore structure and thus increase its life span.